The present invention relates to a device for the optical measurement of the edge to edge or side to side dimension of an object. This dimension can be a thickness, width, diameter, etc. The invention more particularly applies to small objects, for example of a few microns or less.
Inter alia, the invention can apply to the measurement of the width of a pattern deposited or etched on a flat substrate. More particularly, the invention can be used for the precise, correct measurement of a width of a line deposited on a semiconductor substrate. This line can be used as a test pattern for checking and regulating the production process of an integrated circuit. The invention makes it possible to measure sufficiently small patterns for it to be used in the production of micron and even submicron integrated circuits.
Devices exist for measuring the dimensions of objects measured in a few microns. These devices operate on the basis of the principle of a microscope-type optical system, in which a magnified image of the object is formed. An operator directly performs the measurement on the basis of this image. However, this measurement is imprecise, because the contours of the object are poorly defined and the operator must subjectively evaluate the position of these contours. Thus, the measurement varies among individual operators.
There are also devices using photosensitive elements (e.g., video cameras) placed in the plane of the magnified image of the object. These photosensitive elements supply electrical signals which can be automatically interpreted with a view to supplying a measurement of the object.
Although these devices are more accurate than those previously described, because they do not involve human evaluation, which is a source of risks and problems, their accuracy still remains mediocre. Thus, as the contours of the image are poorly defined, it is not possible to accurately know what the photosensitive device is measuring. Careful calibration only makes it possible to obtain satisfactory results if the objects have a size exceeding two microns.
In order to improve the accuracy and correctness of the measurements in the micron range, the existing tendency is to improve optical means in order to increase their quality (elimination of aberrations of all types). Thus, as the optical image is of a better quality, its interpretation is facilitated.